This invention relates to an improvement in a method for extruding a powder material to produce an article. The method includes the steps of passing the material in the form of a mixture which includes a vehicle, through an extruder and then through a die, with the improvement being the step of passing the material through a gear pump before the material enters the die. The term gear pump as used in the present invention can refer to a gear pump having one or more pair of gears in succession in a housing; or it can refer to more than one gear pump arranged in succession. Use of the gear pump helps to stabilize and control the extrusion process. Use of a gear pump allows a high pressure differential to be created in the material from input end to the output end of the gear pump. Therefore highly filled systems can be extruded smoothly to form uniform high quality products.
In the extrusion of filled systems of powder material, such as those used for catalyst supports, for example, ceramic honeycombs, considerable pressure is required in order to force the material through the extrusion die. At present, this is achieved with a hydraulic ram extrusion press, or a two stage de-airing single auger extruder, or in a twin screw mixer with a die assembly attached to the discharge end. In the latter, the proper screw elements are chosen according to material and other process conditions in order to build up sufficient pressure to force the batch material through the die.
There are disadvantages associated with these pressure creating devices especially in extrusion of ceramic materials. For example, in the case of the ram extruder, the process is an intermittant one and there is no means to remove any inhomogenieties in the materials. Also upon the reloading of the extruder with a new charge of material, an interface is formed between the remaining materials and the new charge. This can and does create defects in the material. In the case of a single screw extruder, the material moves in plug flow, and where it is in contact with the screw and walls of the extruder, the material sees considerable shear. If a die is attached directly to the single screw, the material due to rheology differences will extrude differently across the face of the die. In a twin screw mixer used as an extruder, this effect will be decreased. However, in both types of extruder there will be a pulsation in pressure due to the batch coming off the screws. This pulsation affects the quality of the extruded piece. In the case of a single screw extruder, it is difficult to generate high pressure needed to extrude thin walled cellular structures. The twin screw mixer can generate the required pressures but the high pressure results in considerable wear of the screw elements and barrel walls. Also the work input required to generate the pressure in the twin screw mixer increases the batch temperature which changes the extrusion characteristics of the batch. In an effort to overcome screw memory and pulsation, an orifice is often used between the extruder and die. While this can be effective, it requires an increase in pressure from the extruder and does not remove the pulsation from the screw tips.
It would be advantageous therefore, to have an improvement in an extrusion process for powder material in which high pressures are easily generated, and at the same time, in which the above described disadvantages are eliminated and extrusion conditions are stabilized and controlled to consistently produce high quality products, for example, honeycombs.